alter integer primary key to auto_random with certain auto_random_base

[zz-jason]

Feature Request

Is your feature request related to a problem? Please describe:

Let's say we have the following table whose primary key is an integer:

create table t (
	a bigint auto_increment primary key,
	b bigint
);

or column a is defined without auto_increment:

create table t(
	a bigint primary key,
	b bigint
);

After inserting some data to table t, users may detect that there is a hot write region in the table t. Currently, users have to rebuild the table like the following:

• define a new table whose primary column is defined as auto_random
• rewrite all the data from the old table to the new table
• continue to insert data to the new table, which may not have the hot write region issue

Describe the feature you'd like:

Since TiDB has auto_random property for an int primary key, it would be great to support altering the int primary column to auto_random without rebuilding the whole table. Since there is existing data in the table, it's better to set the auto_random_base to be larger than a certain value.

This issue requests to support a certain alter table statement like the following:

alter table t modify a bigint auto_random auto_random_base=?;

Describe alternatives you've considered:

N/A

Teachability, Documentation, Adoption, Migration Strategy:

N/A

[tangenta]

If the performance of this DDL does not matter, it is possible to internally select max(a) from t and rebase to the value. So the syntax will be

alter table t modify a bigint auto_random;